What does drag mean in reference to a cattle drive?

It means the last part of the herd that is being herded. Often the drag has animals that may go off on their own away from the herd, and one or two horsemen have to herd these rogue animals back to the herd. The drag portion of the cattle drive is considered a more advanced position to be in, not a place where greenhorns or tenderfeet should ride nor help out with the drive.